The invention relates to a ballast for operating electric lamps according to the preamble of patent claim 1.
The invention relates in particular to a modern electronic ballast for operating electric lamps which has, in addition to an inverter and its control device, also a microcontroller for monitoring and controlling the functions of the ballast and the lamps operated on it. The microcontroller also makes bidirectional communication possible with a central control unit which is arranged outside the ballast and serves for the central closed-loop or open-loop control of a room lighting system, usually comprising a plurality of luminaires and consequently also a plurality of ballasts.
The European laid-open application EP 0 564 895 A1 describes an electronic ballast for the operation of low-pressure gas-discharge lamps. This ballast has an inverter, a DC voltage supply circuit for the inverter, a load circuit, connected to the inverter, for supplying voltage to an electric lamp, and a device for supplying voltage to the control circuit of the inverter. As already mentioned above, in addition to the control circuit of the inverter, modern electronic ballasts have a microcontroller, which serves for monitoring and controlling the operation of the ballast and the lamps connected to it and makes bidirectional communication possible with a control unit arranged outside the ballast. For these tasks, the microcontroller requires a constant voltage supply, which functions even in what is known as the standby mode of the ballast, that is to say in particular still supplies the microcontroller with electrical energy to an adequate extent when the lamps are switched off and the inverter is deactivated. The device disclosed in the laid-open application EP 0 564 895 A1 for supplying voltage to the control device of the inverter is unsuitable for this purpose.
It is the object of the invention to provide for a generic ballast for the operation of electric lamps an improved voltage supply for the microcontroller which on the one hand reliably supplies the microcontroller with electrical energy, in particular even in the standby mode the ballast, and which on the other hand causes as little power loss as possible.
This object is achieved according to the invention by the features of patent claim 1. Particularly advantageous configurations of the invention are described in the dependent patent claims.
The ballast according to the invention has an inverter, a DC voltage supply circuit for the inverter, a load circuit, connected to the inverter, for supplying voltage to one or more electric lamps, a microcontroller for monitoring and controlling the operation of the ballast or the lamps and a voltage supply device for the microcontroller. According to the invention, the voltage supply device of the microcontroller is designed as a DC-DC converter, the voltage input of which is connected to the voltage output of the DC voltage supply circuit of the inverter and the voltage output of which is connected to the supply voltage input of the microcontroller. The DC-DC converter makes it possible to generate the supply voltage for the microcontroller directly from the AC line voltage rectified by the DC voltage supply circuit, to be precise independently of the operating state of the inverter and the lamps. What is more, the electrical power consumption of the ballast in the standby mode can be lowered to approximately 0.5 W.
The voltage output of the DC-DC converter is advantageously additionally connected also to the supply voltage input of the control device for the inverter switching means. The components of the DC-DC converter are advantageously dimensioned in such a way that an adequately high voltage for supplying both the microcontroller and the control device of the inverter is available at its voltage output. The DC-DC converter can therefore be additionally used also for supplying voltage to this control device. The DC-DC converter advantageously has a step-down converter or is designed as a step-down converter, because a low-volt DC voltage, as required for operating the microcontroller, can be generated with the aid of a step-down converter in a simple way and with relatively low power loss from the comparatively high output voltage of the DC voltage supply circuit. The voltage output of the step-down converter is advantageously formed by an electrolytic capacitor, in order to be able to make an adequately high supply current, of for example 5 mA, available for the control device of the inverter for a short time period, of for example 200 ms. The switching means of the step-down converter and the closed-loop control device for controlling its switching operations are advantageously designed as an integrated circuit. As a result, a space-saving arrangement of the step-down converter or the device for supplying voltage to the microcontroller is made possible. In particular, the main components of the step-down converter, that is to say the switching means and its closed-loop control device, can be integrated into the open-loop control device of the inverter, designed as an ASIC (Application-Specific Integrated Circuit). In order to be able to set the voltage drop across the voltage output of the step-down converter to the desired value, means for temporarily interrupting the switching operations of the switching means of the step-down converter are provided. These means advantageously comprise a threshold switch, which is arranged in a feedback branch to the closed-loop control device of the step-down converter. With the aid of the threshold switch, the voltage drop across the voltage output of the step-down converter can be monitored in a simple way and the closed-loop control device for the step-down converter can be temporarily deactivated or activated. Particularly well suited as the threshold switch is a Zener diode, by the dimensioning of which, and possibly with the aid of a suitably dimensioned additional voltage divider, the output voltage of the step-down converter can be set to the desired value. The aforementioned feedback branch is advantageously electrically isolated from the source potential by means of an optocoupler, in order to exclude any influence of the constantly changing source potential on the switching operations of the step-down converter switching means. In order to avoid radio interference by the step-down converter, the step-down converter is advantageously equipped with radio-interference suppression means. A small-signal diode and a resistor are advantageously used as radio-interference suppression means, the anode of the diode being connected to the step-down converter inductor and its cathode being connected to the positive terminal of the DC voltage output of the step-down converter, and the resistor being arranged in such a way that one terminal is connected to the anode of the small-signal diode and the step-down converter inductor and the other terminal is connected to the frame potential. The small-signal diode, which has only a very low barrier-layer capacitance, has the effect of blocking the step-down converter inductor at the end of its discharge operation and the resistor damps the still remaining natural oscillation of the circuit, which occurs with every transition of the step-down converter diode from the conducting state to the blocking state.